Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Deferred Tax Assets, Net	$ 2,002	$ 1,902
Deferred Tax Assets, Valuation Allowance	230	288
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	103	1,760
Income Tax Expense (Benefit)	180	(1,531)	$ 60
Swiss Federal Tax Administration (FTA) [Member]
Deferred Tax Assets, Net		1,840
Deferred Tax Assets, Valuation Allowance		131
Reduction in Taxes [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability		393
Income Tax Expense (Benefit)	$ 180	$ 80